An image forming apparatus that forms an image electrophotographically such as laser printers includes a photosensitive drum, on which an electrostatic latent image is formed, and a developing cartridge that develops the electrostatic latent image formed on the photosensitive drum.
The developing cartridge includes a developing roller and a supply roller for supplying toner to the developing roller. One sidewall of the developing cartridge includes a gear device unit for driving the developing roller and the supply roller. The gear device unit includes an input gear, to which driving force from a main body of the apparatus is input, a developing roller driving gear, which is attached to an end portion of a developing roller shaft of the developing roller and which meshes with the input gear, and a supply roller driving gear, which is attached to an end portion of a supply roller shaft of the supply roller and which meshes with the input gear. In other words, the developing roller driving gear attached to the end portion of the developing roller shaft of the developing roller and the supply roller driving gear attached to the end portion of the supply roller shaft of the supply roller are meshed with the same gear teeth of the input gear, to which driving force from the main body is input.
When forming an image, driving force is input to the input gear from the main body, so that the input gear is rotated. As the driving force is transmitted to the developing roller driving gear and the supply roller driving gear from the input gear, the developing roller is rotated via the developing roller driving gear and the supply roller is rotated via the supply roller driving gear.